Aircraft brakes are generally multidisc systems in which fixed discs and movable discs are placed in contact in order to generate, by friction, a retarding power. The friction materials constituting the discs or the friction pads placed on said discs are required to re-group a number of properties which will enable them to withstand the thermal and mechanical stresses to which they are exposed, and to have a friction coefficient which is reproducible and stable in the field and the various conditions of use, as well as a strictly controlled abrasion factor.
Refractory composite materials and in particular carbon-carbon composite materials are, among the recently developed friction materials, those which are best suited for the aforesaid application.
The carbon-carbon composites which have a very high specific heat, increasing rapidly with the temperature and this up to above 2000.degree. C., and show strong thermo-mechanical strength against thermal stresses and fatigue, stand up well to friction on themselves. This has prompted the advantageous introduction in multidiscs systems of monobloc friction discs made entirely of carbon-carbon.
Yet, the coefficient of friction of carbon-carbon composites on themselves remains small at temperatures as low as about 100.degree. C. At that temperature, the friction coefficient changes and reaches a value which it will keep in average up to very high temperatures of between 1500.degree. C. and 2000.degree. C., despite rather high perturbations. Moreover, the coefficient of friction of carbon-carbon composites on themselves is sensitive to the environmental conditions, and in particular to the action of humidity. There is also a low static friction coefficient which, being lower than the dynamic friction coefficient, can be an impediment in applications requiring prolonged stops.
Moreover, although the carbon-carbon composites have volume abrasion factors with respect to the absorbed energy which, compared to those of the conventional materials, are relatively low inside a wide range of operation, oxidation is an appreciable cause of increase of those factors, at temperatures exceeding 400.degree. C. to 500.degree. C. The oxidation effect is all the more pronounced that the temperature is high and that the dwelling time at that temperature is long.